I. Field of the Invention
The present invention relates generally to the field of electrical grounding devices and more particularly, to a device which may be inserted into a radio frequency coaxial cable system to render surge protection to sensitive electronic devices and to ground the system in a rapid and inexpensive manner, and to the method of utilizing the invention.
II. Description of the Prior Art
In systems such a coaxial cable television distribution (CATV) systems, there are numerous sensitive electronic devices located at various locations within the system that are highly susceptible to power surges and the like from whatever source. Not only may surges be introduced into the system from local power sources, but it is quite common that cable systems are knocked out of service due to nearby lightning strikes, especially during the peak storm periods. In most cases lightning strikes will cause outages due to burned out amplifiers and fuses, and surveys have shown that outages are a substantial cause of subscriber dissatisfaction. Typically the outage has to be repaired as rapidly as possible, and normally under extremely poor conditions by both regular and standby personnel of the cable television company. If a lightning storm is of long duration then it is quite conceivable that there will be multiple areas in a system which will experience an outage thereby complicating the repair procedure. It is not unusual to have lightning strikes which will induce currents into an electrical conductor measuring several thousand amperes.
There are essentially three ways to protect equipment and outside structures from lightning. The first is a remedial approach wherein lightning strokes are accepted, and the equipment of interest is either designed to withstand the surges or is protected by devices which aid in dissipating the surge energy after a stroke has occurred. The present invention is of this first remedial class. The second is a preventive approach wherein devices are employed to prevent the static charge from building up and, therefore, prevent the possibility of a lightning stroke before it occurs. A system taking this preventive approach is described in U.S. Pat. No. 4,679,114 by Carpenter. This solution employs means to sense the build-up of atmospheric charge and, in response, actively causes a countercharge to build up on a strategically placed electrode thereby neutralizing the atmosphere's static charge in its vicinity and preventing a local lightening stroke. A third approach is to affix lightening attractive conductors at high points over a structure to be protected. These lightening attractive conductors or "lightening rods" are directly connected to earth by heavy, low resistance, low inductive down conductors. This third system provides its protection by preferentially attracting and safely dissipating direct lightening strokes, thus keeping such strokes from directly hitting the protected structure. The invention of Lefort et al (U.S. Pat. No. 4,752,854) is an improved device of this class. These latter two approaches have been used effectively in protecting isolated structures such as radio towers and radar antennaas, but they are not practical for widely distributed systems such as cable television systems. In the case of an outside distribution trunk cable system, prevention is impractical due to the vast area which is covered by the distribution system. It is also necessary since the cable lines are normally installed substantially below power company lines which usually intercept lightening strokes before they directly hit the co-axial cable television cable. Moreover, such atmosphere charge neutralizing and lightening rod systems are completely ineffective in responding to current surges which are caused by power line faults and power system switching transients. These latter surges can occur quite independent of lightening conditions, and are common causes of damage to sensitive communications amplifiers in cable systems.
Therefore, the remedial approach is the most likely approach of this invention in preventing damage to cable television installations. There have been previous devices which have attempted to solve the problem, but typically they have had faults which render the devices only marginally effective. Many previous devices such as represented by U.S. Pat. Nos. 4,939,618 by Fingerson et al, and 5,001,587 by Clark employ conventional air spark gaps which break over at a relatively high voltage to shunt high energy lightning surges to ground. Such devices are characterized by a high degree of variability in their breakover voltage, which variability increases with age and service. Further, their breakover voltages are high enough that sensitive cable trunk line amplifiers and the like can be damaged before they operate. While useful in protecting rugged systems such as electric fences and railroad signals, they are inadequate for application to coaxial cable television distribution systems.
One particular prior art device which has been used in cable television systems employs a power surge suppressor installed on the a.c. power line side of the power inserter, but not on the center conductor of a cable line. Therefore, the SCR type switches used can only protect what surges come from the main trunk line power supply and nothing else. It has been found that these prior art suppressors often fail to protect the RF signal amplifiers along the coaxial cable line. In addition, the prior art device is also mounted on a printed circuit board within the power insertion device in a manner which provides a poor path to ground with limited current carrying capability. In these prior devices, the surge current is conveyed from the circuit board through mounting screws into the base, and therefrom to the cover of the case through screws which attach the cover. There is generally a moisture sealing gasket between base and cover, thereby confining most of the current through the high resistance stainless steel cover screws. From the cover, the current is carried to the cable support strand through the strand mounting clamp on the top of the cover. It is apparent that this is a circuitous path for the surge current to follow with many opportunities for high contact resistances to develop due to ageing, corrosion, and movement due to wind. Experience has been that many of these suppressor are themselves damaged by the events against which they are designed to protect, often by internal arcing or conductor burn-out within their cases. Further, the prior art device cannot be directly connected across the radiio frequency (hereinafter referred to as "RF") signal carrying center conductor of the coaxial cable because of its high RF loading and resulting insertion loss. While it might afford some protection of the amplifiers if so connected, its presence on the line would attenuate the primary transmission signal excessively, thereby rendering the cable system useless.
What is needed, and what has not yet been provided by the prior art, is a cable line surge suppressor which can be connected directly to the RF signal carrying center conductor without inducing significant insertion loss, and which will effectively shunt lightning and power system induced surges to ground, without allowing damage to sensitive amplifiers on the line, and without sustaining damage to itself.